Thanks for the interesting question, Duncan. The product you are using as your source of bacteria lists Lactobacillus plantarum as the only bacterial ingredient in this beverage. Lactobacillus plantarum is a facultative heterofermentative lactic species. This means that under anaerobic conditions, Lactobacillus plantarum behaves like a homofermentative lactic species and produces lactic acid as its sole metabolic by-product, which then switches to a heterofermenter under aerobic conditions and also produces ethanol, acetic acid, and carbon dioxide. Not all Lactobacillus plantarum strains behave the same, but they generally ferment a wide array of carbohydrates (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2546631/) and wort is a great growth media for these bacteria. Sounds like you are concerned that these little critters are going to consume too much fermentable sugars and cut into the production of ethanol by yeast?
The hydrometer is a pretty handy brewing tool and you can certainly use it to monitor the change in wort density before and after your souring phase. The specific gravity of lactic acid solutions vary by concentration, but over the range seen in wort/beer the specific gravity is not much higher than water. This means that as carbohydrates are metabolized the specific gravity of the wort will drop, and that the wort density at the time of yeast pitching can be used like the original gravity (OG) of non-sour brews. This is not an exact measurement, but it is a reasonable approximation.
The hydrometer is a pretty handy brewing tool and you can certainly use it to monitor the change in wort density before and after your souring phase.
One thing to keep in mind about fermentations, in general, is that fermentation by-products often regulate fermentation. In the case of lactic acid fermentations, environmental pH affects metabolism. This is why yogurt contains lactose, even after prolonged storage in the presence of lactic acid bacteria. Kettle sours tend to bottom out around pH 3.2, empirically suggesting that the environment is regulating the rate of fermentation. The other practical consideration relevant to your question is the kettle sour process. Most brewers monitor pH during the souring phase and use pH as a control point; for example, if the goal is to reduce pH to 3.4, the brewing process advances to the next step when this goal is achieved. This means that even if lactic acid bacteria are able to consume all fermentable carbohydrates, the brewing process control would prevent that from occurring. The brewers I know who are commercially brewing kettle sours all carefully monitor pH because many of these brewers are looking for flavor consistency as well as efficient use of their equipment; as soon as the wort pH falls to the target level, they are moving on to wort boiling.
I am not even going to touch attempting to calculate alcohol content of kettle sours because there are simply too many variables to consider. The best calculation tools used to approximate alcohol content in beer based on OG and FG are based on lots of data from “normal beer.” In order to develop the same sort of model for sour beers requires a matrix of process measurements in addition to lab analysis of the ethanol content of the finished beer. Sounds like a great project for an eager brewing chemist!
Something to consider about these beers is how they stand up on the palate and if there is a real need to boost OG to offset the loss of fermentables during the souring phase. Most kettle sours being produced commercially have average to below-average alcohol concentrations and generally are made from worts in the 10–12 °Plato (1.042–1.050 SG) range, and sometimes lower. Berliner weisse is a great example of a low-alcohol beer that does not leave my palate asking for the missing experience that is often the case with reduced-alcohol, non-sour styles. If you like what you find in the market and want to brew these sorts of beers at home, skip the additional fermentables. Alcohol consumption is dropping globally and brewers are being challenged to produce lower-alcohol alternates to traditional beers. Kettle sours, with their refreshing acidity and high drinkability, are really a style that fits in with this shift. Focus your process and recipe on how your beer stands up to tasting, and make changes from a flavor standpoint first and ABV targets second.